With the increase in subscribers to portable telephone services, effective use has become a greater problem than that of frequency. One technique for solving this problem is CDMA (Code Division Multiple Access). CDMA cellular phone systems adhere to TIA/EIA/IS-95.
In CDMA communication systems, four rates are obtained for use as the data rate for transmitting information bits. Namely, the four data rates of 8.6 kbps, 4.0 kbps, 2.0 kbps and 0.8 kbps are used. Actually, information bits are transmitted using a specified one of the four data rates. The information bits to be transmitted are divided into one frame every 20 msec, and are transmitted every frame. As a result, with the above mentioned four data rate, 172 bits, 80 bits, 40 bits or 16 bits are respectively included in one frame. Among the four data rates, 12 CRC bits and 8 CRC bits are respectively added to the information bit frames of 8.6 kbps and 4.0 kbps. CRC bits are not added to the information bit frames of 2.0 kbps and 0.8 kbps. 8 tail bits are then added to the frames of each data rate. At this time, the data rates respectively become 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps. Next, coding is carried out to cut the rate in half. After this, the data rate of each of the information bits is made uniform at 19.2 kbps by repeating the same symbol twice in the case of a data rate of 4.8 kbps, 4 times in the case of a data rate of 2.4 kbps, and 8 times in the case of a data rate of 1.2 kbps. The information bits that have thus had the data rates made uniform are next interleaved, and then scrambled using a long code PN sequence. A power control bit is also embedded in the information bits that have been scrambled. With CDMA transmission systems, 384 symbols are included in one frame. One frame is split into 16 power control groups (referred to hereinafter as PCG). Accordingly, 24 symbols are included in one PCG. Within the 24 symbols of each PCG, power control bits of 2 symbols are arranged at random positions as power control information to be transmitted to a mobile station by a base station. At this time, the symbols that were originally in those positions are lost. The position at which the power control bits are arranged is inside the 2 symbol power control bits, and is defined as a header symbol position. This position is based on 4 bits extracted from the aforementioned long code PN sequence, and 16 positions are obtained. Because one power control group is comprised of 24 symbols, the power control bits are arranged from the header to the 17th symbol, and are not arranged in the symbols from the 18th to the 24th. As a result, the information bits in which the power control bits are embedded are scattered by a Walsh function and a pilot long code PN sequence, then QPSK modulated and transmitted to a mobile station.
The signal transmitted from the base station is received at the mobile station. This received signal is QPSK demodulated, then reverse scattered by a Walsh function and pilot long code PN sequence. The power control bits are also extracted. `0` is inserted at the positions from which the power control bits have been extracted, to indicate that there is no information. As a result, a 19.2 kbps received symbol code sequence is obtained. Next, this received symbol code sequence is decoded and a CRC check is carried out. The received signal is a reduction encoded signal, decoding is carried out using, for example, Viterbi decoding. At first, the data rate of the received signal cannot be determined at the mobile station. Because of this, it is necessary to estimate the data rate. That is, there is a need to promptly estimate the data rate of the actually transmitted signal from the four possible data rates.